AUTOMATIC TRANSMISSION FLUID:
The owner's manual specifies Dexron II automatic
transmission fluid for the GM400 transmission. Draining this
fluid and refilling with Type F fluid will result in firmer
shifts. The Type F fluid is less slippery.

Alternatively, there are aftermarket fluids available
that are less slippery than either Dexron or Type F,
resulting in firmer shifts yet.

If you have the early XJ-S with the Borg-Warner
automatic, the owner's manual specifies Type F fluid.
However, there are reports that some Jaguar dealers refill
these transmissions with Dexron II anyway, because they feel
their customers appreciate the smoother shifts resulting
from the slipperier fluid. This might not be a good idea,
since -- in any automatic transmission -- firmer
shifts cause less wear.

RAISING THE LINE PRESSURE: Tim Blystone sends this
idea, in response to the above suggestion of using Type F
fluid: "The F-Type fluid is somewhat thinner and is supposed
to allow the tranny to shift faster and more positively. It
does no harm and neither have I ever been able to see any
good it does either. I would rather use Dexron 3, remove the
pump and replace the pump pressure spring with a new red
spring. Used in Corvettes. This will raise line pressure and
make the whole thing shift more positive."

GM400 TRANSMISSION SHIFT POINT MOD #1: The 5.3
liter Jaguar V12 was designed prior to the fuel crunch of
1973 and has "over-square" engine dimensions -- the bore (90
mm) is considerably larger than the stroke (70 mm). Such a
design is conducive to high performance at high RPM and is
therefore ideal for racing applications, but results in poor
low end torque. However, to optimize fuel economy, it is
best if an engine is turning rather slowly, so the stock
GM400 automatic transmission is set up to keep the V12 in
the RPM range where its performance is poorest.

The point at which the GM400 shifts is determined
primarily by a governor that responds to engine RPM, and is
varied by two things: 1) a vacuum modulator that makes the
car upshift promptly when vacuum is high (light throttle);
2) an electrical switch on the throttle linkage that forces
the tranny to downshift, and remain in lower gears until
5000 RPM, when the pedal is pressed all the way to the floor
("kickdown").

The vacuum modulator is a bolt-on device on the right
side of the transmission. Some of these modulators
are adjustable by removing the vacuum hose and inserting a
straight blade screwdriver into the vacuum fitting. Turning
the adjustment changes the preload on the spring, therefore
altering the shift points.

The vacuum modulators are available in several sizes, and
since they are a standard GM part, are found in any auto
parts store for reasonable prices. In general, the smaller
they are, the more vacuum required to make the tranny
upshift, so the longer the car will remain in the lower
gears. Of course, it is recommended that you make sure to
buy one that is adjustable.

Rather than buying a smaller modulator, Greg Meboe
modified his existing one: "I needed to increase the spring
pressure against the diaphragm. I did this by collapsing the
modulator body around part of the spring in a vise,
effectively shortening the working length of the spring. I
imagine I took about 30% of the travel out of the spring.
This had the effect of increasing the effective
spring rate. The vacuum modulator modifications produce an
upshift between 500 and 1500 rpm's higher than before,
depending on throttle position."

GM400 TRANSMISSION SHIFT POINT MOD #2: For
absolute maximum performance, shift points should be
selected that surround the peak on the horsepower curve --
without exceeding the redline, of course. The engine should
be allowed to run past the power peak, so that when shifting
the engine doesn't drop back too far below the power
peak.

The published stats rate the Jaguar V12 at 5000 RPM. With
the large steps between gears on a three-speed transmission,
the engine should be taken to the redline of 6500 before
shifting to optimize performance.

The GM400 slushomatic transmission, of course, was
designed with the 1950's-design pushrod V8 in mind. Not only
does it normally shift at relatively low rpm, it absolutely
refuses to allow the engine to rev beyond 5000 rpm;
it will shift at this point even if you have selected low
gear with the lever. The above modification to the vacuum
modulator will not affect the limit where the GM400 will
force a shift.

The governor assembly is contained under a sheet metal
cover on the right rear side of the transmission, and there
is an access cover under the carpet of the XJ-S to get to
it. If you remove the governor and grind a little weight off
of each of the counterweights, the engine will shift at
higher RPM under all conditions. This modification will
allow the engine to run faster than 5000 RPM, where the
stock GM400 will force a shift no matter what.

GM400 TRANSMISSION SHIFT POINT
MOD #3: If you want serious performance from your XJ-S,
install one of the many available "shift kits" for the
GM400. Keep in mind when you select your kit that your
objective is to allow higher RPM shift points at full
throttle (when the kickdown switch on the throttle cable is
activated) or when manually shifting; High RPM shifts when
taking it easy are of little benefit and will hurt fuel
economy and passenger comfort. Many shift kits offer more
firm shifting, some shift at redline at all times (!) and
some even render your car a 3-speed manual.

Most shift kits can be installed by simply lowering the
pan from the transmission and removing the valve body
within. Replace a few springs and the like, and reassemble.
See the note in the Maintenance section regarding the
differences between pre- and post-1988 GM400
transmissions.

Alan Jenks reports on a "Shift Improver Kit' from B&M
Racing to the Turbo Hydromatic 400 automatic transmission.
There are two kits, one for pre-1988 TH400 (B&M #20260)
and one for post-1988 (B&M #20261). The kit can be
fitted in two forms, either ëheavy duty' (stage 1) or
ëstreet/strip' (stage 2). Stage 1 provided firmer
shifting without compromising the quality of the ride, which
is what I wanted."

Per Chad Bolles: B&M makes a product called a
Transpak, "with this you can set up the trans to shift like
you want, from very hard, to normal street." B&M also
makes a Racing Kit, "this is a race very hard shift
kit (neck snapping)." Finally, in what should probably be
listed as GM400 Transmission Mod #3A, there is a replacement
valve body. "The B&M valve body changes the shift
pattern to 123NRP instead of PRN321."

Jan Wikström had his local tranny shop "lock up the
freewheel on 1st gear, which gives me respectable engine
braking and a better "feel" in the really low-speed twisty
bits. Caution: it's now possible to engage 1st at any
speed..."

There are torque convertors available that have a higher
"stall speed", which sorta means they engage at a higher
RPM. This is wonderful for the Jaguar V12, but does require
removing the tranny to change. According to Thomas E.
Alberts, "B&M tells you to try for 500-750 RPM less than
the RPM at peak torque."

Michael Kenrick reports that the torque convertor from a
3.8 Buick Regal Turbo can be used to provide a more suitable
stall speed.

According to Chad Bolles, the later model XJ-S uses the
HI-Stall convertor. "It is the same converter used in the
427 L-88 and LS6 and LS7 Chev, flash stalls around
2600-2800rpm, the difference being the Jaguar converter has
6 lugs on it where the Chev has 3." He adds that a Chevy
convertor may be used in the Jag, leaving the other three
bolt holes unused. Reportedly, some GM torque convertors use
six bolts as well.

Bolles also explains the difference between stall speed
and "flash stall". Stall speed is the RPM the engine runs at
continuously if the brake is held and the accelerator is
floored in gear. Note: this is very hard on the equipment,
and causes a lot of heat in the transmission; such tests
should only be done for a couple seconds, and the car should
be driven around at normal speeds for a while afterward to
allow the transmission fluid cooler in the radiator to cool
the transmission back down. "Flash Stall, hold the brake,
trans in gear, quickly mash the acc pedal, watch the Tach,
it will flash to about 2500-2800 rpm, but if you just hold
the brake and ease the acc pedal down the converter will not
reach the same rpm before it tries to spin the rear tyres.
Don't ask me why, I just know that that's how it works."

GM400 RATIO CHANGE: Thomas E. Alberts says, "...I
agree that the standard gearset is poorly matched to the
XJ-S... Note that the modified gearsets, like TCI's, lower
the second gear ratio as well as first. Observe:

TH 400

TCI gearset

1st

2:48

2:75

2nd

1:48

1:57

3rd

1

1

I think it would make a very nice improvement and without
any sacrifice of top end speed."

ADDING OVERDRIVE TO THE GM400: John Goodman has
heard about "overdrive units manufactured by American
Overdrive Inc. designed for autoboxes, GM400 included? This
effectively changes your GM400 into a six speed.

"Two possibilities here would give much better
acceleration. The unit can be supplied with the internals
reversed so you have underdrive first, underdrive second and
underdrive top. Or install a lower axle ratio and leave the
overdrive in and switch it out for "Sport Mode"."

GM400 REPLACEMENT: The shift points, cruise speed
RPM, fuel economy and top end can all be optimized by
replacing the GM400 with a 5- or 6-speed manual
transmission. A major task, requiring the installation of
flywheel, bell housing, clutch, clutch pedal, console
modifications, etc. Changing the final drive ratio
(differential gears) should be considered, since 5- and
6-speeds generally have an overdrive top gear and the final
drive ratio is already tall enough. AJ6 Engineering reports
that the ECU needs the overrun cutoff disabled "to avoid
driveline shunt at low speed."

The author of this book has performed this
modification on his car, using a 5-speed Borg-Warner NWC
transmission from a Camaro. If you desire to know more about
such a project, you are welcome to call. Chad Bolles of
Jaguar South has also done such projects, and has prepared a
5-speed kit. Also, Gran Turismo Jaguar now offers kits for
5- and 6-speed conversions.

General info for those considering such mods follows,
much of it provided by Mike Frank. There are four
transmissions commonly considered. The first is the
Borg-Warner NWC 5-speed, which comes in Camaros, Mustangs,
and several other vehicles; it has an integral shifter (no
external linkage), and there are a couple different
locations for this shifter, reportedly the Mustang
configuration is better for the XJ-S than the Camaro
configuration. This tranny is adequate for a stock XJ-S V12
but may be marginal if extensive engine mods are done. There
are lots of ratios available.

There are 5-speed conversion kits using the "JT5"
transmission, which is supposedly the same thing as a BW NWC
only different. Only conversions for E-types and other older
Jaguars are mentioned, but since the E-type SIII V12 is
included, it may be possible to figure something out for the
XJ-S. JT5 kits are offered by Terry's Jaguar and
Vicarage.

The second possibility is the 6-speed used in the Dodge
Viper and the latest Firebird models. It has some advanced
features such as carbon composite synchros. This is a
massive tranny, and will probably handle whatever a Jag V12
can be made to dish out. There is a wide variety of ratios
available; GT Jaguar reportedly offers a kit with suitable
ratios to allow use of the stock 2.88:1 final drive
ratio.

Third possibility is a Getrag 5-speed. It is supposedly
also adequate for all conceivable uses. The Getrag is a
novelty in the US, but is reportedly fairly common in
Europe. SNG Barratt offers kits using this transmission;
again, only E-types are mentioned, but the SIII V12 is
included.

Fourth is the Toyota Supra "oval case" 5-speed. It might
be from a Japanese six-cylinder car, but it has nevertheless
been used with considerable success in Jaguars with either
6- or 12-cylinder engines. This transmission is described as
"an all-alloy case 5-speed and can be distinguished by 9
bolts and 2 dowels holding the box to its bellhousing." If
you get your own tranny from a junkyard, note that before
installing it is recommended to remove the front cover of
the transmission which holds the input shaft bearing and the
input end of the layshaft and replace the light-duty
layshaft bearing with a generic industrial-duty bearing of
the same size -- a five-minute job.

More info on the Supra tranny from Dellow, a major
supplier of these kits: "There are 4 gearing variations
available:

W55

W57

W58

W59

1st

3.57

3.28

3.28

3.95

2nd

2.06

1.89

1.89

2.14

3rd

1.38

1.27

1.27

1.27

4th

1

1

1

1

5th

0.85

0.86

0.78

0.85

There are four possible stick positions: 18", 19", 20
1/2" and 21", as measured from the front of the box. Note
that the W57, W58, and W59 may be difficult to locate, with
the W58 described as "very rare".

Dellow also offers some Supra 4-speed boxes; ratios are
3.57/2.06/1.38/1.00.

There is also some history of the Toyota "steel case"
5-speed from Celicas, Crowns, Coronas, Cressidas, etc.,
being used in Jaguars with success, even though it is
described as not quite as strong as the Supra. This tranny
has 7 bolts holding it to the bellhousing.

Also note that, despite the fact that Dellow is located
in Australia, getting all the hardware needed from them
might still be cheaper than buying such things as the
transmission itself locally, once the exchange rates,
shipping, etc., are all figured in.

John Napoli tells us about Tilton: "They cater to the
racers. They sell a full range of bellhousings for the V12
that accept popular manual transmissions. You can even get a
small-diameter, multi-plate clutch with a small,
reverse-rotation, gear reduction starter that mounts
backwards on the bellhousing, alongside the
transmission. Imagine -- starter changes in the XJ-S without
removing exhaust pipes, and from inside the car!! Be still
my heart."

Jaguar made a manual transmission XJ-S -- with the AJ6
engine. Still, maybe some parts would be helpful for the V12
conversion. Chad Bolles reports, "The pedal box from the 5sp
XJ-S will fit any XJ-S, but the problem is it will only work
on an XJ-S with ABS, and I have found no way to make it work
on a non-ABS car." Aaargh! Note that that AJ6-powered XJ-S
existed in Europe before the advent of ABS brakes, so
perhaps there is still a possibility there.

FINAL DRIVE RATIO CHANGE: There are many ratios
available to fit the Jag final drive. However, be aware that
if the ratio is changed significantly, the differential
carrier will also have to be replaced. Since the lower
ratios (higher numbers) require a smaller pinion, the ring
gear needs to move closer to center to mesh properly. This
could be accomplished by making the ring gear thicker or
adding a spacer, but instead a replacement carrier is used
with the mounting flange in a slightly different position.
Unfortunately, the carrier costs more than the gears.

Please see page the secion on the Drivetrain
for other considerations on the final drive unit.